1. Field of the Invention
The present invention relates to a receiver for radio communications and to a method for demodulating reception signals, and more particularly, to a receiver suitable for a radio communications system wherein the transmission speed and modulation method are variable.
2. Description of the Related Art
Of currently used radio telephone methods and radio paging systems, there are known systems wherein the transmission speed and modulation method can be varied. For example, in the FLEX (registered trademark) system, which is a radio paging system, there are three transmission speeds (1600/3200/6400 bps) and two modulation methods (2FSK (Frequency Shift Keying)/ 4FSK), and two symbol rates are set (1600/3200 sps). In a radio paging system of this kind, in order to minimize noise bandwidth, desirably, the pass bandwidth of an LPF (Low Pass Filter) located in the signal path after quadrature detection is changed in accordance with the symbol rate.
However, in conventional receivers, if the aforementioned LPF is formed by an analogue circuit, for example, then the LPF is designed to be compatible with a symbol rate of 3200 sps. Therefore, if the symbol rate is 1600 sps, problems arise, such as increased susceptibility to noise and deterioration of the bit error rate (BER).
In order to resolve problems of this kind, the output signal from the aforementioned LPF is converted to binary form, and BER degradation is reduced by passing the signal again through a moving average-type digital LPF which changes bandwidth according to the symbol rate.
However, the moving average-type filter comprises numerous stages of shift registers. Therefore, the circuit becomes relatively large in size, and power consumption also increases. In particular, in radio telephones and pagers of recent years, it has been sought to reduce power consumption and to increase call time and standby time, but if a moving average-type filter is used as described above, then it is difficult to achieve increases in call time and standby time, due to the scale of the circuit involved.